recurrence within ICR: a seismicity-based model (given a
1 second, and that of Atkinson and Boore extends to a
weight of 0.25); and a geology-based model (given a weight
period of 2 seconds. The EPRI (1993) relationship was
of 0.75). The seismicity-based model divides ICR into cells
extrapolated to a period of 2 seconds. This was
of one-half degree latitude and longitude and calculates
accomplished by extrapolating the coefficients of the
recurrence rates based on the historical seismicity in the
attenuation relationship and examining the reasonableness
cell. Different degrees of smoothing of seismicity rates and
of the resulting spectral prediction. The smooth quadratic
b-values among adjacent cells is accomplished using the
form of the relationship of Atkinson andBoore (1995)
methodology developed by EPRI (1988). In the geology-
underestimates their simulations of longer period ground
based model, Zone ICR is divided into subzones as
motions at distances beyond 100 km (62 miles). Therefore,
indicated in Figure E-23. Different combinations of
their relationships were modified at periods greater than
subzones are defined in a logic tree approach. The possible
0.5 second to result in ground motion estimates closer to
combinations are controlled in part by the presence or
the simulation results. Plots of the attenuation
absence of four possible tectonic boundaries within the ICR
relationships of EPRI (1993) and Atkinson andBoore
(Figure E-23) and the assessed likelihood that these
(1995) for peak ground acceleration and response spectral
features represent fundamental boundaries that control the
accelerations at 1.0 second are presented in Figure E-26.
distribution, rate, and maximum magnitudes of seismicity.
The modifications to the 1-second motion at distances
The logic tree for weights assigned to these boundaries is
greater than 100 km (62 miles) can be seen in the figure.
shown on Figure E-25. Thirty alternativesubzonations
The plots in Figure E-26 clearly indicate the distinctive
(not shown herein) of ICR result from the logic tree of
differences between the two eastern United States
Figure E-25. Within each subzone of each alternative,
attenuation relationships: the Atkinson andBoore (1995)
seismicity rates are determined based on the seismicity
relationships result in higher spectral values than those of
within the subzone and assuming the rate is uniform within
EPRI (1993) for peak ground acceleration and for short-
the subzone.
period response spectral accelerations (less than about
0.2 second period), but lower values than those of EPRI
(c) Probabilistic distributions of maximum
(1993) at longer periods.
earthquake magnitudes are also part of the source model
logic tree. These probabilistic distributions were
(c) In the hazard analysis, the relationship of
determined using the methodology developed by EPRI
EPRI (1993) was given a higher weight (0.67) than that of
(Johnston et al., 1994) that utilized worldwide data bases to
Atkinson and Boore (1995) (0.33). The reason for this
assess maximum earthquake magnitudes in stable
judgment was that the EPRI (1993) relationship resulted
continental regions (like the eastern United States (EUS))
from an EPRI study that involved input from a number of
where active faults have not been identified and therefore
ground motion experts and thus could be viewed as having
maximum magnitude cannot be estimated on the basis of
achieved a certain degree of consensus regarding the
fault dimensions (as is done in the western United States
model. The practical effect of higher weighting on the
(WUS)). However, for the New Madrid zone, maximum
EPRI (1993) model is to increase longer period ground
earthquake magnitudes were estimated on the basis of both
motions and reduce short-period ground motions.
(1) estimated rupture models by Johnston (1996) and
Gomberg and Ellis (1994) and correlations of magnitude
(3) PSHA Results. Hazard curves obtained from
with rupture dimensions, and (2)estimates of magnitudes
the analysis for peak ground acceleration and response
of the 1811-1812 earthquakes by Johnston (1996).
spectral acceleration at two periods of vibration are shown
in Figure E-27. The uncertainty bands around the mean
(2) Ground Motion Attenuation Characterization.
curves, reflecting the alternative seismic source models and
attenuation relationships incorporated into the logic tree,
(a) It was desired to estimate ground motions
are shown in the figure. The contributions to the hazard
on rock at the site. Two attenuation relationships
are almost entirely from Zone ICR. Figure E-28 shows
applicable to hard rock in the EUS for horizontal peak
contributions within ICR from large New Madrid Zone
ground acceleration and response spectral accelerations of
earthquakes with rates defined by paleoseismic data
ground motions at different periods of vibration were used.
(dashed-dotted line) and smaller earthquakes defined by
The relationships are those of EPRI (1993), (later published
seismicity (dashed line). It can be seen that the smaller
as Toro et al., 1997) and Atkinson and Boore (1995) (later
earthquakes dominate hazard at higher frequencies
published as Atkinson andBoore, 1997).
(probabilities) of exceedance and the larger, 1811-1812-
type earthquakes dominate at lower frequencies
(b) The relationship for response spectral
(probabilities) of exceedance. Figure E-29 compares the
acceleration of EPRI (1993) extends to periods as long as
hazard obtained from geology-based and seismicity-based
E-31
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